Gaseous mixtures comprising hydrogen (H2) and carbon monoxide (CO) can be converted into a hydrocarbon product stream by a catalytic process known as Fischer-Tropsch synthesis (F-T synthesis). The most common catalysts useful in F-T synthesis (“F-T catalysts”) are based on Fe and/or Co, although Ni- and Ru-based catalysts have also been described (see e.g. U.S. Pat. No. 4,177,203; Commereuc (1980) J. Chem. Soc., Chem. Commun. 154-155; Okuhara (1981) J. Chem. Soc., Chem. Commun. 1114-1115). Generally, Ni-based catalysts are relatively more selective for producing methane whereas Co-, Fe- and Ru-based catalysts are more selective for hydrocarbons having at least two carbon atoms (C2+ hydrocarbons). Moreover, the selectivity for C2+ hydrocarbons can be increased by decreasing the H2:CO ratio, decreasing the reaction temperature and decreasing the reactor pressure.
It has been previously described that unsupported cobalt-manganese oxide catalysts can be used as an F-T catalyst having an improved selectivity for C3 hydrocarbons and a suppressed CH4 selectivity (see Van der Riet (1986) J. Chem. Soc. Chem. Commun 798-799 and Keyser (1998) Applied Catalysis 171:99-107). The unsupported cobalt-manganese oxide composition suitable for use as F-T catalyst was produced by a process comprising the steps of co-precipitating cobalt and manganese oxides from cobalt- and manganese-comprising solution, calcining the precipitate to form a calcined catalyst precursor and reducing the calcined catalyst precursor to obtain the cobalt-manganese oxide catalyst composition (see Colley (1988) Ind. Eng. Chem. Res. 27:1339-1344). It was found that the catalyst precursor comprised the mixed spinels Co2MnO4 and Co2Mn2O4. Reduction of the catalyst precursor resulted in an unsupported catalyst composition comprising metallic Co, MnO and a certain amount of mixed spinels Co2MnO4 and Co2Mn2O4.
It was the disadvantage of the prior art that F-T synthesis of hydrocarbon products result in a mixture of undesired products and less desired olefin products. A major drawback of conventional unsupported cobalt-manganese oxide F-T catalysts is their relatively low activity resulting in a relatively low syngas conversion rate.
It was an object of the present invention to provide an improved Fischer-Tropsch catalyst (F-T catalyst) having superior catalyst activity while maintaining high hydrocarbon selectivity and low carbon dioxide (CO2) and methane (CH4) selectivity. In another aspect, it was an object of the present invention to provide an improved process for producing a higher yield of ethylene and propylene from syngas in addition to value added products, such as 1-hexene and 1-butene.